Generally, a secondary battery refers to a rechargeable battery, while a primary battery refers to a non-rechargeable battery. Secondary batteries are widely used for electronic devices such as cellular phones, notebook computers, video cameras, electric vehicles, or the like. In particular, a lithium secondary battery has an operating voltage of about 3.6 V, triple the capacity of nickel-cadmium batteries or nickel hydrogen batteries generally used as power sources of electronic devices, and due to its high energy density per unit weight, are being utilized more and more.
The lithium secondary battery generally uses lithium oxide and carbonaceous material as cathode active material and anode active material, respectively. The lithium secondary battery includes an electrode assembly in which a cathode plate and an anode plate respectively coated with the cathode active material and the anode active material are disposed with a separator being interposed between them, and an exterior material which seals and receives the electrode assembly together with an electrolyte.
Meanwhile, the lithium secondary battery may be classified into a can type secondary battery where the electrode assembly is included in a metal can and a pouch type battery where the electrode assembly is included in a pouch of an aluminum laminate depending on the shape of the battery case. In addition, the can type secondary battery may further be classified into a cylindrical battery and a rectangular battery depending on the shape of the metal can. The exterior material of the rectangular or cylindrical secondary battery includes a battery can and a cap assembly sealably coupled to an open end the battery can.
FIG. 1 is a sectional view showing a cap assembly 10 of a conventional cylindrical secondary battery.
Referring to FIG. 1, the cylindrical secondary battery generally includes a cylindrical battery can 20, a jelly-roll type electrode assembly 30 received in the battery can 20, a cap assembly 10 coupled to the upper portion of the battery can 20, a beading portion 40 provided at the front end of the battery can 20 for mounting the cap assembly 10, and a crimping portion 50 for sealing the battery.
The electrode assembly 30 is wound into a jelly-roll shape in a state where a separator is interposed between a cathode and an anode. A cathode lead 31 is attached to the cathode and connected to the cap assembly 10, and an anode lead (not shown) is attached to the anode and connected to the lower end of the battery can 20.
The cap assembly 10 includes a top cap 11 forming a cathode terminal, a safety element 12 such as a Positive Temperature Coefficient (PTC) element for intercepting a current by increasing the resistance when the temperature in the battery increases, a safety vent 13 for intercepting a current and/or for exhausting gas when the pressure in the battery increases, a first gasket 14 for electrically separating a safety vent 13 except for a specific portion from a current intercepting member 15, and a current intercepting member 15 connected to a cathode lead 31 connected to the cathode, which are laminated in the above order. In addition, the cap assembly 10 is mounted to the beading portion 40 of the battery can 20 in a state of being mounted to a second gasket 16. Therefore, in a normal operating condition, the cathode of the electrode assembly is electrically connected to the top cap 11 via the cathode lead 31, the current intercepting member 15, the safety vent 13, and the safety element 12. However, if gas is generated from the electrode assembly 30 due to overcharging or the like to increase the inner pressure, the safety vent 13 inverts its shape to protrude upward, and at this time the safety vent 13 separates from the current intercepting member 15 to intercept the current. Therefore, the charging/discharging work does not proceed any further, thereby ensuring the safety of the secondary battery. Further, if the inner pressure of the secondary battery increases over a certain value, the safety vent 13 fractures, and the pressurized gas exhausts through a gas hole formed in the top cap 11 via the fractured portion, thereby preventing the battery from exploding.
However, in the above conventional cap assembly 10, in a case where an impact is applied from the outside to the side of the battery can 20, the safety of the secondary battery may not be ensured. For example, in a case where an impact is applied to the side of the secondary battery, the battery can 20 may deform to bend inwards, and at this time the current intercepting member 15 may separate from the first gasket 14 to contact the battery can 20. Generally, the current intercepting member 15 is connected to the cathode, and the battery can 20 is connected to the anode. Therefore, if the current intercepting member 15 contacts the battery can 20 as mentioned above, a short may occur at the inside of the secondary battery, which may cause problems such as fracturing, firing or exploding of the secondary battery.